294 PHYSIOLOGY 



CHAP. 



eye is not obliged to use its mechanism of accommodation in 

 order to see objects distinctly. But if the object comes nearer 

 than this the plane on which the image is formed is too far away 

 from the sensitive layer of the retina ; accordingly vision by 

 simple static refraction is indistinct, and it is necessary to bring 

 dynamic refraction into play in order to see distinctly. 



It was long held that accommodation in the eye took place as 

 in the photographic camera, which is adapted to different distances 

 by shifting the sensitive plate to a greater or less distance from 

 the lens. That is, the retina was thought capable of forward and 

 backward movement, by means of the external oculo-motor 

 muscles. But it was subsequently recognised that another, more 

 perfect mechanism controlled the accommodation of the eye. 



Descartes (1636) first suggested that distinct vision of objects 

 at different distances depends on the power of the eye to alter the 



form of the lens. But the ob- 

 ^ jective proof of this theory was 



^ I c . a, & a on ty discovered two centuries 



later by M. Langenbeck,Cramer, 

 and Helmholtz (1849-53). 



As we saw (Sanson-Purkinje 

 images, p. 286), the length of 

 the radii of curvature of mirror - 

 images reflected from the spheri- 

 cal surfaces of the cornea and 



FIG. 132. To show the alterations in Purkinje's Igrig can be 

 images when the eye passes from static to 



dynamic refraction, in accommodation. (After accurately from the size of 



Helmholtz.) a, image reflected from the cornea ; . T p , .-, , . 



b, from the anterior ; c, from the posterior surface the image. II, Willie Observing 



nea^viS: A ' ***** ****** vision ' *' durins these images, the subject is told 



to focus a near object it will be 



seen that the image reflected from the cornea does not alter, while 

 the image from the front of the lens, on the contrary, becomes 

 much smaller, showing that the convexity of the mirror increases 

 during accommodation. The image reflected from the back of the 

 lens also becomes smaller, but in so slight a degree as to be un- 

 important. The experiment is easier if the image of two luminous 

 squares is reflected from the eye instead of the image of a candle 

 flame. It is then seen (Fig. 132) that the two images from the 

 front of the lens are not only reduced, but are brought together, on 

 accommodation. 



Knapp (1860) determined on four eyes the position of the near 

 and the far points, and the curvature and position of the cornea 

 and surfaces of the lens in distant vision and during accommoda- 

 tion for near vision, and found that the alteration in the curvature 

 of the lens suffices to explain the increased refractive power of the 

 eye in focussing a near object. 



During the change in form of the lens its posterior surface 



